Embodiments of the invention relate generally to synthetic jet actuators and, more particularly, to synthetic jet actuators having an element therein for lowering the structural natural frequency thereof.
Synthetic jet actuators are a widely-used technology that generates a synthetic jet of fluid to influence the flow of that fluid over a surface. A typical synthetic jet actuator comprises a housing defining an internal chamber. An orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected in an external environment out from the orifice of the housing. Examples of volume changing mechanisms may include, for example, a piston positioned in the jet housing to move fluid in and out of the orifice during reciprocation of the piston or a flexible diaphragm as a wall of the housing. The flexible diaphragm is typically actuated by a piezoelectric actuator or other appropriate means.
Typically, a control system is used to create time-harmonic motion of the volume changing mechanism. As the mechanism decreases the chamber volume, fluid is ejected from the chamber through the orifice. As the fluid passes through the orifice, sharp edges of the orifice separate the flow to create vortex sheets that roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity. As the mechanism increases the chamber volume, ambient fluid is drawn into the chamber from large distances from the orifice. Since the vortices have already moved away from the edges of the orifice, they are not affected by the ambient fluid entering into the chamber. As the vortices travel away from the orifice, they synthesize a jet of fluid, i.e., a “synthetic jet.”
Referring to FIGS. 1-3, a synthetic jet actuator 10 as known in the art, and the operation thereof, is shown for purposes of describing the general operation of a synthetic jet actuator. The synthetic jet actuator 10 includes a housing 11 defining and enclosing an internal chamber 14. The housing 11 and chamber 14 can take virtually any geometric configuration, but for purposes of discussion and understanding, the housing 11 is shown in cross-section in FIG. 1 to have a rigid side wall 12, a rigid front wall 13, and a rear diaphragm 18 that is flexible to an extent to permit movement of the diaphragm 18 inwardly and outwardly relative to the chamber 14. The front wall 13 has an orifice 16 of any geometric shape. The orifice diametrically opposes the rear diaphragm 18 and connects the internal chamber 14 to an external environment having ambient fluid 39.
The flexible diaphragm 18 may be controlled to move by any suitable control system 24. For example, the diaphragm 18 may be equipped with a metal layer, and a metal electrode may be disposed adjacent to but spaced from the metal layer so that the diaphragm 18 can be moved via an electrical bias imposed between the electrode and the metal layer. Moreover, the generation of the electrical bias can be controlled by any suitable device, for example but not limited to, a computer, logic processor, or signal generator. The control system 24 can cause the diaphragm 18 to move periodically, or modulate in time-harmonic motion, and force fluid in and out of the orifice 16. Alternatively, a piezoelectric actuator could be attached to the diaphragm 18. The control system would, in that case, cause the piezoelectric actuator to vibrate and thereby move the diaphragm 18 in time-harmonic motion.
The operation of the synthetic jet actuator 10 is described with reference to FIGS. 2 and 3. FIG. 2 depicts the synthetic jet actuator 10 as the diaphragm 18 is controlled to move inward into the chamber 14, as depicted by arrow 26. The chamber 14 has its volume decreased and fluid is ejected through the orifice 16. As the fluid exits the chamber 14 through the orifice 16, the flow separates at sharp orifice edges 30 and creates vortex sheets 32 which roll into vortices 34 and begin to move away from the orifice edges 30 in the direction indicated by arrow 36.
FIG. 3 depicts the synthetic jet actuator 10 as the diaphragm 18 is controlled to move outward with respect to the chamber 14, as depicted by arrow 38. The chamber 14 has its volume increased and ambient fluid 39 rushes into the chamber 14 as depicted by the set of arrows 40. The diaphragm 18 is controlled by the control system 24 so that when the diaphragm 18 moves away from the chamber 14, the vortices 34 are already removed from the orifice edges 30 and thus are not affected by the ambient fluid 39 being drawn into the chamber 14. Meanwhile, a jet of ambient fluid 39 is synthesized by the vortices 34 creating strong entrainment of ambient fluid drawn from large distances away from the orifice 16.
A drawback of existing synthetic jet designs, such as that shown and described in FIGS. 1-3, is the noise generated from operation of the synthetic jet. Audible noise is inherent in the operation of synthetic jets as a result of the flexible diaphragm being caused to deflect in an alternating motion, and the natural frequencies of the synthetic jet's various operational modes (structural, disk-bending, and acoustic) impact the amount of noise generated during operation. According to existing designs, a structural natural frequency of a synthetic jet may reach levels of 600 Hz, resulting in a high level of audible noise being generated, which is highly undesirable.
Another drawback of existing synthetic jet designs is the amount of power consumed during operation of the synthetic jet. A high structural natural frequency of the synthetic jet corresponds to a higher amount of power that is needed to be provided to the synthetic jet to deflect the diaphragm. High rates of power consumption not only increase the cost of operating the synthetic jet, but also decrease the efficiency of the synthetic jet. For example, when the synthetic jet is used as a cooling device, convection cooling is negatively affected by high rates of power consumption, as such increased power consumption generates unwanted heat.
The noise level and rates of power consumption are both a result of the natural frequency of the synthetic jet's maximum deflection, which in turn is a result of the material properties and shape of components in the synthetic jet actuator. Specifically, the shape of components in existing synthetic jet actuators results is an increased spring constant associated therewith, thereby leading to an increased structural natural frequency of the synthetic jet actuator.
Accordingly, it is desirable to provide a synthetic jet having a low structural natural frequency in order to reduce the amount of noise generated from operation of the synthetic jet and to lower the amount of power consumed during operation of the synthetic jet.